I have a weird problem with my shader.
In the fragment shader I'm reading data from a 3D texture in a for-loop until the correct alpha value is found, with a maximum number of iterations specified as a uniform int (currently 320). The fragment color is then determined by the number of iterations done along with the last RGB read from the texture. So far there's no problem, it works perfectly.
The problem appears when I try to speed things up with a 2D texture of the same width and height as the 3D one. Before the for-loop I read a value from it to determine a starting point for the loop to avoid unnecessary texture lookups.
What happens is that the shader compiles fine with no warnings, but then it does nothing. Apparently the fragment shader doesn't even get called!
The vertex shader is a simple pass-through one, and there are no other stages. Both textures are of 16-bit float RGBA format. I'm working on a Windows 7 with a GeForce GTX 560 Ti.
If someone could tell me what's wrong with this or if there's an alternative way to do it (there has to be a dynamic loop in the fragment shader though), I would very much appreciate it!
ps. With a correct alpha value I mean a range (as in 0.8<A<0.9).